Allogeneic blood and marrow transplantation remains the only viable cure for children who suffer from many serious non-malignant hematological diseases. Transplantation, however, carries a high risk of fatal complications. Much of the risk stems from the use of high dose radiation and chemotherapy for conditioning, the treatment administered just prior to transplant that eliminates the patients' marrow and immune system, effectively preventing rejection of the donors' cells. Attempts to make blood and marrow transplantation safer for children with non-malignant diseases by using lower doses of radiation and chemotherapy have largely failed because of a high rate of graft rejection.